The study of "Maternal Phenylketonuria" encompasses two major components: I. The mediated transport of L-phenylalanine and L-tyrosine and the possible competitive inhibition mechanisms of aromatic amino acids at the placental level will be studied in pregnant rhesus monkeys. These primates will undergo an operative procedure whereby a polyvinyl catheter will be placed into the common carotid artery of the fetus, which will then facilitate blood sampling (after maternal L-phenylalanine loading studies). II. "Phenylketonuria" will be induced in pregnant rhesus monkeys by means of a high L-phenylalanine content and p-chlorophenylalanine in their diet. Three groups of monkeys will receive specific amounts of such compounds to obtain blood phenyl-alanine levels of 5, 10, and 15 mg/dl respectively in the monkeys. The fourth group will constitute a control group. After delivery the infants will undergo anthropometric examinations and at age 6 and 12 months psychometric studies will take place.